This proposal will test three related hypotheses that attempt to explain how and why inhalation of some, but not all, antigens stimulates an allergic response. These hypotheses suggest that the adaptive immune system will respond to stimulation with a foreign protein plus an inflammatory cytokine, such as IL-1Beta, GM-CSF, or TNF-alpha, with a Th2 cytokine response, while the same stimuli, plus a cytokine that promotes IFN-gamma production, such as IL- 12 or type 1 IFN, will induce a Thl response. As a corollary, we hypothesize that allergens share the property of inducing the innate immune system to produce inflammatory cytokines, but not cytokines that promote IFN-gamma production, while conventional protein antigens (non-allergens) fail either to stimulate inflammatory cytokine production or stimulate production of both inflammatory and Th1-promoting cytokines. Results of studies of cytokine production in worm-infected mice support the additional hypothesis that Th2 cytokine responses, but not Th1 responses, are self-sustaining (i.e., a Th2 response will persist in the absence of continued inflammatory cytokine production, while a Th1 response will switch to a Th2 response in the absence of continuing Th1-promoting cytokines). These hypotheses will be tested with a series of studies in which: 1) mice are inoculated either tracheally or intraperitoneally with allergens, conventional antigens +/- inflammatory and/or IFN-gamma-inducing cytokines, or toll-like receptor ligands that induce distinct patterns of cytokine production; 2) cytokine responses are measured by a novel in vivo cytokine capture assay and/or real-time RT-PCR; 3) pulmonary allergic responses are evaluated by quantitating IgE, IgG1, and IgG2a antibody responses, eosinophilia, goblet cell hyperplasia, and mouse mast cell protease 1 levels; 4) "knockout" mice and neutralizing anti-cytokine antibodies are used to evaluate the ability of allergens to induce an allergic response in the absence of inflammatory cytokines; and 5) a gene chip approach is used to determine whether allergens rapidly induce the expression of characteristic set of genes, other than or in addition to inflammatory cytokine genes, that are not activated by conventional antigens. Results of these studies may explain why some protein antigens are allergens, improve our understanding of why some pathogens induce a Th1 response while others induce a Th2 response, and promote new and better strategies for vaccination procedures that would induce the optimal cytokine response to prevent or control infection by a particular pathogen.